Skip to main content
Create interactive lessons using any digital content including wikis with our free sister product
. Get it on the
Pages and Files
Adding and Subtracting Fractions
Adding and Subtracting Mixed Numbers
Comparing and Ordering Decimals
Comparing and Ordering Numbers
Conversions Between Mixed and Improper Fractions
Dividing by 10,100, 100
Dividing Fractions and Mixed Numbers
Estimating Sums and Differences
Add "All Pages"
Prime and Composite
How to do Prime and Composite Numbers
Composite numbers are number that have more than one factor pair.
24 is an example: 4x6=24, 8x3=24, and 2x12=24. 24 is a composite number because there is more than one set of numbers that can be multiplied together to make 24. Composite numbers must have more than one factor pair. If they don't they are prime.
3 is an example. 1x3=3 Three is a prime number because only one factor pair makes three.
If you want to know if a number is prime or composite there is an easy way to check. All of the even numbers, except for 2, are composite. This means that if the number you are checking is not two and it is even then it is composite. If it is odd then we have to check it out more carefully.
Most prime numbers are odd, but not all of them. 3x3=9 so nine is a composite number. Here are some steps to help you decide which odd numbers are composite. If the number is odd you do not need to check for any even divisibility so you can forget about checking for 2, 4, 6, 8, and 10. That leaves only 3, 5, 7, and 9 to check. Three is easy, just add up all the digits in the number and if they equal a multiple of 3 you know the number is composite. While you have the sum of the digits you can also check for nine. If the sum is a multiple of nine then it is composite. If you know the nines finger rule you can check it that way also.
The next test is 5, which is also easy, of the number you are checking ends in 5 then it is composite.
Last is the check for seven. Double the last digit and subtract it from the digits that are left. If the answer is a multiple of 7 then the number is composite. This rule is tricky, if you need help with it
Once you have worked through all this you can guarantee you will know id the number is prime or composite.
Prime Number Calculator
help on how to format text
Turn off "Getting Started"